This invention relates to a pneumatic tire with a rubber tread which utilizes an alpha-methyl styrene oligomer.
Pneumatic rubber tires are sometimes intended to be used for purposes where traction (skid resistance) is a primary consideration. For example, it may be desired for a high performance tire tread rubber to have good initial grip, or traction, while also having good durability and, also, good overall traction or grip. Often for such tires, resistance to tread wear is less of a consideration.
Historically, for emphasizing tread rubber traction, the tread may, for example, be compounded to have a relatively high ratio of resin and processing oils to rubber (e.g. a range of about 15 to about 40 weight percent in the rubber composition) which may be more conventionally expressed in terms of parts by weight per 100 parts by weight rubber (phr) as being, for example, in a range of about 45 to about 120 phr.
The purpose of the resin, among other purposes, is to enhance traction of the tread due, at least in part, to a choice of resin with an appropriate softening point.
For example, a resin with a softening point of about 30xc2x0 C. would be expected to soften and become very hysteretic at a tread temperature of about 20xc2x0 C. to about 50xc2x0 C. and, thus, aid in providing tire traction at such tread temperatures.
A considerably higher softening point resin would be desirable to enhance tread traction at a higher tread temperature (e.g. at least 100xc2x0 C.) associated with the higher vehicular speed.
However, the resin may become too incompatible with the rubber and lose the tire traction properties.
Therefore, it is desirable to use a resin that is at least semi-compatible with the rubber.
Various examples of such resins which, as it is understood, are or may be used to aid in tire tread traction are hydrocarbon-derived synthetic resins, coumarone-indene resins, rosin, rosin derivatives and dicyclopentadiene based resins such as, for example, dicyclopentadiene/diene resins.
Such resins may typically have softening points (Ring and Ball) within the aforesaid range of about 20xc2x0 C. to about 110xc2x0 C. and even up to about 170xc2x0 C.
In another aspect of tire tread rubber considerations, it should be pointed out that viscoelastic properties of a rubber, or a rubber blend, for tire tread applications, are important. For example, a tangent delta viscoelastic property is the ratio of the viscous contribution to the elastic contribution for a viscoelastic rubber article subjected to a cyclic deformation. The term xe2x80x9ctangent deltaxe2x80x9d is often referred to herein as xe2x80x9ctan. deltaxe2x80x9d. Its characterization of viscoelastic properties of rubber is well known to those skilled in such art. Such property is typically represented in the form of a curve as a temperature sweep plot of tangent delta values on a y, or vertical, axis versus temperature on an x, or horizontal, axis.
Although various rubber compositions are taught to provide various benefits, some for tire treads, it is desired to provide a pneumatic tire having a rubber tread having enhanced traction qualities over a relatively wide temperature range and, thus, a tan. delta characteristic which maintains a high tan. delta as the temperature increases.
In the description of this invention, rubber compound, sulfur-cured rubber compound, rubber composition, rubber blend and compounded rubber are used somewhat interchangeable to refer to rubber which has been mixed with rubber compounding ingredients. Such terms are well known to those having skill in such art.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d is an abbreviation for xe2x80x9cparts (of an indicated material) by weight per 100 parts by weight rubberxe2x80x9d.
In accordance with this invention, a pneumatic tire is provided having an outer circumferential tread where said tread is comprised of, based upon 100 parts by weight rubber (phr),
(A) 100 parts by weight of at least one diene hydrocarbon based elastomer;
(B) from about 1 to about 40, alternately about 3 to about 10, phr of a liquid alpha-methyl styrene oligomer;
(C) about 20 to about 150, alternately about 60 to about 100 phr of reinforcing filler selected from at least one of carbon black, precipitated silica and silica modified carbon black; and
(D) a coupling agent for said silica and/or silica modified carbon black, as the case may be, having a moiety reactive with said silica, (usually hydroxyl groups on the surface of the silica such as silanol groups) and carbon black and another moiety interactive with at least one of said diene-based elastomers.
It is to be appreciated that said tread rubber composition may also contain
(E) about one to about 50 phr of at least one resin having a softening point within a range of about 65xc2x0 C. to about 165xc2x0 C. (Ring and Ball softening point) selected from at least one of:
(i) coumarone-indene resin(s);
(ii) alkylated hydrocarbon resin(s);
(iii) aromatic petroleum hydrocarbon resin(s); and
(iv) dicyclopentadiene/diene resin(s).
A significant aspect of this invention is that the liquid resin greatly reduces the dynamic low-strain stiffness (Gxe2x80x2 at one percent strain), and gives a comparable high strain loss compliance (Jxe2x80x3 at fifty percent strain) to the control resin.
This is considered herein to be important because it tends to give tire traction at lower temperatures (Gxe2x80x2 at one percent strain) before tread heat-up can occur in the tread and, subsequent, improved cornering traction (Jxe2x80x3 at 50 percent strain) at higher tread temperatures.
The alpha-methyl styrene oligomer for use in this invention may be characterized as coupled alpha-methyl styrene moieties that tend to form in a head-to-tail fashion and may have a residual olefinic character.
The oligomer is composed primarily of dimers as may be represented by the following Formula (I) 
wherein n is a value from zero to 100, alternatively an average of from 2 to 10.
Such oligomers may be prepared, for example, by free radical, cationic or anionic polymerization as would be understood by one having skill in such art.
By tread traction it is meant handling, grip and cornering performance of the tire and particularly the tire""s resistance to skidding on a road surface.
As hereinbefore discussed, where traction is to be emphasized for a tire tread, various resins may be desired to be included in its rubber composition.
The term xe2x80x9csoftening pointxe2x80x9d for the resins, as hereinbefore observed, refers to Ring and Ball softening point as determined by ASTM No. E28.
Representative examples of such contemplated resins are petroleum hydrocarbon resins, coumarone-indene resins, alkylated petroleum hydrocarbon resins, aromatic hydrocarbon resins, dicyclopentadiene/diene resins, and rosin and rosin derivatives.
Preferred contemplated resins are of the coumarone-indene type, dicyclopentadiene/diene type, and aromatic petroleum resins.
For example, for a coumarone-indene resin in a softening point range of about 20xc2x0 C. to 40xc2x0 C., a resin such as Cumar R-29 from Neville Chemical Co. is contemplated. Coumarone-indene resins are a class of resins recognized by those having skill in such resin art. They are typically derived from the polymerization of coumarone and indene.
For example, for an alkylated petroleum hydrocarbon resin in about a 120xc2x0 C. to 150xc2x0 C. softening point range, primarily saturated alkylated resins are contemplated such as, for example, Nevchem 150 by Neville Chemical Co. Such resins might be prepared, for example, by the alkylation of aromatic hydrocarbons with dicyclopentadiene (see U.S. Pat. No. 3,023,200).
For example, for an aromatic petroleum hydrocarbon resin in an intermediate softening point range of about 90xc2x0 C. to about 110xc2x0 C. a resin such as LX-782 by Neville is contemplated. In one aspect, such resins contain carbon-to-carbon unsaturation (double bonds) and may conventionally be a mixture of aromatic and acyclic polymer moieties, although they may be referred to as xe2x80x98aromatic petroleum resinsxe2x80x9d. The aromatic component of the resin is preferably selected from styrene, alpha-methylstyrene or t-butyl styrene and the remaining component of the resin is an aliphatic hydrocarbon. Such class of resins are believed to be recognized as such by those having a skill in such resin art.
A dicyclopentadiene/diene resin composition might be contemplated as being a reaction product of the polymerization reaction between dicyclopentadiene and at least one olefin hydrocarbon (a diene) copolymerizable therewith which has 4 to 12 carbon atoms and which is selected from monoolefins and diolefins. While various diolefins are contemplated, including limonene and cyclooctadiene, cyclooctadiene is preferred. Preferably, such dicyclopentadiene-olefin copolymer is comprised of about 50 to about 80 weight percent dicyclopentadiene. Such resin is contemplated which has a softening point in a range of about 20xc2x0 C. to about 170xc2x0 C. or even up to about 220xc2x0 C. Thus, on a preferable basis, for example, a dicyclopentadiene/cyclooctadiene copolymer resin composed of about 50 to about 80 weight dicyclopentadiene is contemplated.
Representative of such various dicyclopentadiene/ diene resins are shown in U.S. Pat. No. 3,927,144 and, for such disclosure, is hereby incorporated herein by reference.
In the compounding of the tire tread, various rubbers may be used which are, preferably, relatively high unsaturation diene-based rubbers. Representative examples of such rubbers are polymers and copolymers of dienes such as, for example, isoprene and 1,3-butadiene and copolymers of such dienes with styrene and/or alpha-methyl styrene, preferably styrene.
Representative of such rubbers are, although they may not be so limited, for example: styrene/butadiene rubber, cis 1,4-polyisoprene rubber (natural and synthetic) cis 1,4-polybutadiene rubber, acrylonitrile-butadiene rubber, styrene/isoprene/butadiene rubber, styrene/isoprene rubber, high vinyl polybutadiene rubber having a vinyl content of from 40 to 90 percent, and 3,4-polyisoprene.
It should readily be understood by one having skill in the art that said tread portion of the pneumatic tire, as well as the rubber or other material in the basic carcass, which normally contains reinforcing elements in the tread region, can be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly-used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, stearic acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur-vulcanizable and sulfur-vulcanized materials (rubbers), the certain additives mentioned above are selected and commonly used in conventional amounts.
Such pneumatic tires are conventionally comprised of a generally toroidal-shaped carcass with an outer circumferential tread, adapted to be ground contacting, spaced beads and sidewalls extending radially from and connecting said tread to said beads.
For high performance applications, typical additions of carbon black comprise about 60 to about 140 parts by weight of diene rubber (phr), preferably 70 to 100 phr. Typical amounts of processing aids comprise 40 to 140 phr. Typical amounts of silica, if used, comprise about 10 to about 20 phr and amounts of silica coupling agent, if used, comprise about 0.05 to about 0.25 parts per part of silica, by weight. Representative silicas may be, for example, hydrated amorphous silicas and, particularly, precipitated silicas. A representative coupling agent may be, for example, a bifunctional sulfur-containing organo silane such as, for example, bis-(3-triethoxy-silylpropyl)tetrasulfide and/or disulfide, bis-(3-trimethoxy-silylpropyl)tetrasulfide and/or bisulfide, and bis-(3-trimethoxy-silylpropyl)tetrasulfide and/or disulfide grafted silica. Typical amounts of antioxidants comprise 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as those disclosed in The Vanderbilt Rubber Handbook (1978), pages 344-346. Suitable antiozonant(s) and waxes, particularly microcrystalline waxes, may be of the type shown in The Vanderbilt Rubber Handbook (1978), pages 346-347. Typical amounts of antiozonants comprise 1 to about 5 phr. Typical amounts of stearic acid comprise 1 to about 3 phr. Typical amounts of zinc oxide comprise 2 to about 5 phr. Typical amounts of waxes comprise 1 to about 5 phr. Typical amounts of peptizers comprise 0.1 to about 1 phr. The presence and relative amounts of the above additives are not an aspect of the present invention which is primarily directed to the utilization of specified blends of resins in tire treads as sulfur-vulcanizable compositions.
The vulcanization is conducted in the presence of a sulfur-vulcanizing agent. Examples of suitable sulfur-vulcanizing agents include elemental sulfur (free sulfur) or sulfur-donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur-vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur-vulcanizing agents are used in an amount ranging from about 0.5 to about 8 phr with a range of from 1.5 to 2.25 being preferred.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally, a primary accelerator is used in amounts ranging from about 0.5 to about 2.0 phr. In another embodiment, combinations of two or more accelerators in which a primary accelerator is generally used in the larger amount (0.5 to 1.0 phr), and a secondary accelerator which is generally used in smaller amounts (0.05-0.50 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of such accelerators have historically been known to produce a synergistic effect of the final properties of sulfur-cured rubbers and are often somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are less affected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Representative examples of accelerators include amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The tire can be built, shaped, molded and cured by various methods which will be readily apparent to those having skill in the art.
In the practice of this invention, the tread rubber containing the resin blend can be integral with and adhered to various tire carcass substrate rubber compositions. Typically, such a rubber composition is at least one of a styrene-butadiene copolymer rubber, cis 1,4-polyisoprene (natural or synthetic) rubber, 3,4-polyisoprene rubber, 1,4-polybutadiene rubber, styrene-butadiene-isoprene rubber and styrene-isoprene rubber. Optionally, such a blend for a portion of the tread, particularly where the tread is in the region of the sidewall area of the tire may contain one or more of butyl rubber, halobutyl rubber, such as chlorobutyl or bromobutyl rubber, and ethylene/propylene/conjugated diene terpolymer rubber, polyisoprene and polybutadiene rubbers.
In the further practice of this invention, the tread can typically be applied in the building of the green tire in which the uncured, shaped tread is built onto the carcass following which the green tire is shaped and cured.
Alternately, the tread can be applied to a cured tire carcass from which the previous tread has been buffed or abraded away and the tread cured thereon as a retread.
The practice of this invention is further illustrated by reference to the following examples which are intended to be representative rather than restrictive of the scope of the invention. Unless otherwise indicated, all parts and percentages are by weight.